Did vitamin D help New Zealand during the 1918-19 influenza outbreak? Researchers speculate that maybe it did.
The 1918-19 influenza pandemic was unusually widespread and deadly. Across the world, the influenza affected 500 million people and killed 50 to 100 million people. Even more striking was that the pandemic reached every corner of the Earth, from Pacific islands to the Arctic.
Researchers wanted to see, however, if there were any differences in incidence rates among different islands throughout the world. If there were, they wanted to know what kind of differences were there between the islands.
The researchers decided to compare the islands of Iceland and New Zealand, two relatively large islands on opposite ends of the world. The 1918-19 influenza pandemic peaked several times and both these countries were affected at the same peaks, despite being in different hemispheres.
Iceland suffered from a very high incidence rate of 66-90%, while New Zealand was much lower at 30-50%. Furthermore, Iceland had a higher mortality rate during that time, 830 per 100,000, compared to New Zealand, 550 per 100,000.
Paradoxically, when the researchers looked into the public health measures of both nations, they found Iceland had taken more and stricter actions, like firmer restrictions on public gathering.
So why did Iceland have higher incidence and mortality rate? One speculative explanation is vitamin D levels.
The researchers note that when the influenza struck both countries in November, Iceland was in sun-deprived winter and subsequent dropping vitamin D levels, while New Zealand was in summer where vitamin D levels were increasing.
While we’ll never know for sure what the protective factor was, research today shows that vitamin D can protect against respiratory infections and influenza to a certain degree. Was it the culprit in 1918? We can only guess, that yes, maybe it was.